Effect of grain-coating mineralogy on nitrate and sulfate storage in the unsaturated zone
Unsaturated-zone sediments and the chemistry of shallow groundwater underlying a small (∼8-km 2 ) watershed were studied to identify the mechanisms responsible for anion storage within the Miocene Bridgeton Formation and weathered Coastal Plain deposits in southern New Jersey. Lower unsaturated-zone sediments and shallow groundwater samples were collected and concentrations of selected ions (including NO 3 − and SO 4 2− ) from 11 locations were determined. Grain size, sorting, and color of the lower unsaturated-zone sediments were determined and the mineralogy of these grains and the composition of coatings were analyzed by petrographic examination, scanning electron microscopy and energy dispersive analysis of x-rays, and quantitative whole-rock x-ray diffraction. The sediment grains, largely quartz and chert (80–94% w/w), are coated with a very fine-grained (<20 μm), complex mixture of kaolinite, halloysite, goethite, and possibly gibbsite and lepidocrocite. The mineral coatings are present as an open fabric, resulting in a large surface area in contact with pore water. Significant correlations between the amount of goethite in the grain coatings and the concentration of sediment-bound SO 4 2− were observed, indicative of anion sorption. Other mineral–chemical relations indicate that negatively charged surfaces and competition with SO 4 2− results in exclusion of NO 3 − from inner sphere exchange sites. The observed NO 3 − storage may be a result of matrix forces within the grain coatings and outer sphere complexation. The results of this study indicate that the mineralogy of grain coatings can have demonstrable effects on the storage of NO 3 − and SO 4 2− in the unsaturated zone.